In biochemistry, a molecule or substrate that binds with another. Often used in the cell signalling field to describe something that binds with a receptor on a cell surface. Examples include:

          receptor                ligand
 
          IL-2 receptor            IL-2
          Galactose repressor      galactose
          hemoglobin               oxygen
	  fibronectin              heparin

Also used in organometallic and inorganic chemistry to define the molecules attached or surrounding a metal ion. In solution any metal will be cloaked in solvent (water molecules, for example). However, there are also organic species that can chelate a metal, providing some of its ligands.

Haem molecules occupy four ligand positions around the central iron atom, in a plane. When bound to Haemoglobin, the other two (above and below the haem plane) are occupied by a sidechain atom (a histidine) and the oxygen molecule. Therefore, iron-haem complexes are ligands in the biochemical sense for the globin; and the oxygen is a ligand in the chemical sense for the iron! Clear :)